(1) Field of the Invention
The present invention relates to a method for preparing a concrete in which the slump is improved remarkably. The concrete of the present invention is applicable in manufacturing an ordinary concrete or a lightweight concrete which is useful in all concrete structures such as marine concrete structures, buildings, foundations and the like.
(2) Description of the Prior Art
Heretofore, a concrete was usually manufactured by the following method. The method has generally been utilized in a raw concrete factory or elsewhere:
Process (1) In the first place, a cement, water, sand and an admixture material are mixed with one another.
Process (2) Next, a course aggregate (a gravel, a macadam (crushed stone) or a lightweight aggregate for a lightweight concrete) is added and mixed.
The present invention contemplates providing a method for manufacturing a concrete with the aim that the concrete manufactured has improved performance as compared with the performance of an ordinary concrete or a lightweight concrete manufactured by a conventional method.
While the aforementioned improvements in the performance result in a noticeable improvement in the slump of the concrete of the present invention, the strength performance obtained therefor is either equal to or even greater than that of the conventional concrete.
In the case of the lightweight concrete, the lightweight aggregate which is dispersed therein has many voids (the volume of the inner voids occupies 50% of that of the total concrete) and its water absorption is remarkably high, which makes quality control of the concrete very difficult. This high water absorption greatly influences the quality of a fresh lightweight concrete and a hardened lightweight concrete. Of the factors which influences the quality freeze-thaw (freezing and thawing) and pump pressure delivery (transfer) properties are important. In particular, the freeze-thaw property largely varies with a control process of the lightweight aggregate, thereby rendering it difficult to handle such an aggregate. Moreover, with regard to the pump pressure delivery property, it is prohibited by a standard of Japanese Building Society to deliver the lightweight aggregate under pressure by a pump (since the lightweight aggregate is water-absorptive, the pump would be clogged with the aggregate therein, so that the delivery of the concrete would be impossible).
A conventional kneading method of manufacturing the lightweight concrete comprises a first step of uniformly mixing, sand with a cement and adding water thereto to prepare a mortar, and a second step of mixing the mortar with a lightweight aggregate to form the concrete. In this case, however, there is a large amount of free water in the mortar, and upon the addition of the lightweight aggregate thereto, the free water is absorbed thereby resulting in the fluidity of the mortar being described (owing to this phenomenon, the pump pressure delivery of the mortar would be impossible) and the freeze-thaw resistance thereof would also be deteriorated. Alternatively, the lightweight aggregate allowed initially to absorb water to its full capacity, but even in this case, the pressure delivery by the pump can not be accomplished and the freeze-thaw property also becomes poor.
Accordingly, the present invention provides an improved method for preparing a lightweight concrete. According to this method, a kneading process of the lightweight concrete is altered, and in consequence, the above mentioned drawbacks of the conventional technique are solved. That is, the freeze-thaw resistance can be heightened, and the delivery of the lightweight concrete under a pumping pressure can be achieved, as in an ordinary concrete.